In optical fiber communications arrangements, there has been a steadily increasing improvement in the reduction of signal loss within the fiber itself but fiber signal loss is the major contributing factor to signal loss in long distance systems. Fiber losses can be compensated for by the use of fiber amplifiers. One critical loss factor is the quality of the coupling when two signal bearing fibers are connected together, inasmuch as the junction between the two fibers represents a discontinuity where signal losses can, and do occur.
In general, fibers are connected together in end-to-end, butting relationship by means of connectors having fiber holding ferrules therein. It is common practice to finish the end of each ferrule in a flat surface normal to the axis of the fiber whereby the flat faces of the two ferrules involved in a connection bear against each other, and hence, the fiber ends abut, often with an index matching gel therebetween. Such an arrangement is unsatisfactory for several reasons, among which are lack of parallelism of the ferrule end faces, non-concentricity of the fibers in the ferrules, and no physical fiber contact, and the necessity of re-applying index matching gel each time there is a disconnect-reconnect process. One solution to the problem of non-contact, has been to make the ferrule ends, and the fiber contained therein, convex or domed so that actual physical contact between the fibers occurs. Of even greater effect on the connection, however, is the Fresnel reflection at the end surfaces of the fibers, whereby reflected light is fed back within the fiber toward the signal source. Such reflection gives rise to signal loss, instabilities in the signal source, and a deterioration of the signal-to-noise (S/N) ratio.
One solution to the problem of Fresnel reflection is to form the end surface of each ferrule, and hence, the fiber end which is co-planar therewith, at an angle such that the signal reflection angle is greater than the fiber numerical aperture. Thus, when light is reflected at the interface, it does not travel back along the fiber but is, in effect, directed out the sides thereof in the form of leakage. There is, as a consequence, some small signal power loss, but source stability and S/N are improved. Such a beveled arrangement is becoming more and more popular, but it has certain inherent disadvantages, chief among which is the necessity of insuring that the butting angular faces of the two ferrules and their fibers are exactly parallel or as near thereto as can be realized given the normal manufacturing tolerances. For optimum signal transmission, the planar end faces of the fibers should be in full surface contact, and any such misalignment or non-parallelism prevents such contact. In U.S. Pat. No. 4,615,581 of Morimoto, there is shown an arrangement which overcomes, at least to some extent, the foregoing problem. In the arrangement disclosed in that patent, the ferrule end faces are normal to the ferrule axes, but each fiber axis is at an angle to its ferrule axis. Thus, although the fiber end face is coplanar with the ferrule end face, it is at the desired angle relative to the fiber axis to prevent light signal feedback resulting from Fresnel reflection. There still remains, however, the problem of alignment of the fiber ends. Any slight angular offset about its axis of a ferrule relative to the other ferrule can reduce the desired full surface contact of the fiber ends.
Because of the requirement of parallelism between beveled connector ferrule ends discussed in the foregoing, and physical contact requirement, it has been proposed that the ferrule end, and hence the fiber end, be ground to a convex or domed shape, with the high point or vertex of the dome shape coinciding with the optical axis of the fiber. With such a configuration, the faces of the two connecting ferrules abut at their vertices, thereby obviating the necessity of absolute parallelism characteristic of flat faces. When the vertices coincide with the optical axes of the fibers, good contact between fibers is assured, and, because the end faces are still beveled, the feedback from reflections is still minimized. Unfortunately, however, when such a ferrule face is ground to a beveled convex shape, it is not, using present grinding methods, possible to make the vertex of the dome and the optical axis coincide. As will be discussed more hilly hereinafter, the displacement of the vertex from the optic axis is a function of the diameter of the ferrule end, and coincidence occurs only when that diameter is zero, for face tilts other than zero degrees.
In U.S. Pat. No. 5,140,660 of Takahashi, there is shown a ferrule configuration specifically aimed at this problem. The solution proposed by Takahashi is a reduction in diameter of the end of the ferrule so that, when the face is ground in a convex shape, the apex of the dome approaches, but does not coincide with , the optic axis. The amount of the offset of the apex relative to the axis is, however, within acceptable tolerances. One problem with the Takahashi arrangement is that it requires a special ferrule shape having a reduced diameter tip joined to the rest of the ferrule by a tapered section. Such a ferrule "blank" is more expensive to produce than the normal, single diameter blank, which, when considered in light of the vast quantities of fiber optic connectors being manufactured and used at the present time, becomes an important economic factor. In addition, the tapered section, over an extended period of use including frequent connect-disconnect operations, tends to accumulate dirt which can ultimately result in signal loss or decreased S/N ratio. Thus, although the Takahashi arrangement is an improvement over prior art devices, it is relatively expensive, does not assure near perfect or perfect coincidence of the dome vertex with the optic axis, and is subject to, or causes, accumulation of dirt which can interfere with optimum signal transmission.